1. Field of the Invention
The present invention relates to a hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
2. Description of the Related Art
In general, a hydraulic shock absorber attached to a suspension system of a vehicle includes a cylinder having a hydraulic fluid sealed therein, and a piston slidably fitted in the cylinder to define therein two chambers, which are communicated with each other through a communicating passage. The flow of hydraulic fluid in the communicating passage caused by sliding movement of the piston is controlled by an orifice passage and a disk valve mechanism, thereby generating damping force. When the piston speed is relatively low, the hydraulic shock absorber exhibits damping force characteristics (orifice characteristics) in which the damping force changes quadratically with the piston speed by the restrictor action of the orifice passage. When the piston speed exceeds a predetermined level, damping force characteristics (valve characteristics) in which the damping force changes linearly with the piston speed are provided by the action of the disk valve.
Incidentally, for improvement in the ride quality and steering stability of the vehicle, the hydraulic shock absorber is desired to exhibit damping force characteristics as follows: During normal running of the vehicle that is accompanied by fine vibration of the mass under the spring of the suspension system, orifice and valve characteristics which produce relatively small damping force are desired. During a turning or braking operation that is accompanied by relatively slow and large vibration of the mass under the spring of the suspension system, orifice and valve characteristics which produce relatively large damping force are desired.
Under these circumstances, a hydraulic shock absorber has been proposed in which the orifice characteristics on the extension side are changed with the frequency of vibration of the mass under the spring of the suspension system such that when the frequency of the vibration is relatively low, the orifice passage area is reduced to generate relatively large damping force, whereas, when the frequency of the vibration is relatively high, the orifice passage area is increased to reduce the damping force, thereby improving the ride quality and steering stability of the vehicle, as is disclosed, for example, in Japanese Utility Model Public Disclosure (KOKAI) No. 62-25346.
The above-described conventional hydraulic shock absorber suffers, however, from the problem that since only the orifice characteristics on the extension side are changed, no satisfactory improvement can be obtained in the ride quality and steering stability for the contraction side.
In view of the above-described circumstances, the assignee of the present invention has proposed a frequency-sensitive hydraulic shock absorber in Japanese Patent Application No. 2-283852, in which orifice characteristics (alternatively, together with valve characteristics) on both the extension and contraction sides change with the frequency of vibration under the spring of the suspension system.
However, the proposed frequency-sensitive hydraulic shock absorber involves problems as stated below:
In the above-described hydraulic shock absorber, a shutter is displaced by the pressure of hydraulic fluid produced in accordance with the frequency of stroke of a piston rod so as to open and close a bypass passage, thereby controlling the damping force characteristics such that when the frequency is relatively high, relatively small damping force ("soft" characteristics) is generated, whereas, when the frequency is relatively low, relatively large damping force ("hard" characteristics) is generated.
In the above-described hydraulic shock absorber, if the direction of stroke of the piston rod changes when the frequency is relatively low and hence the damping force has become relatively large as a result of the displacement of the shutter, impact is generated because of the "hard" damping force characteristics. In addition, as the shutter moves, the damping force characteristics once change over from "hard" to "soft" and then return to "hard". Consequently, the damping force characteristics change over during the stroke of the piston rod on the extension or contraction side, causing impact to be generated. Thus, the ride quality of the vehicle is degraded.